Not applicable.
Not applicable.
The invention presented here is in the domain of devices for the measurement of the flow rate of liquids in a channel.
The thermocouple according to the invention is designed to be incorporated in a mass flow meter which is placed in a system for the management and control of the circulation of high purity gas, for example.
These mass flow meters usually consist of a capillary tube for the circulation of fluids, on which the representative measurements of the flow are carried out, and which is arranged in parallel with the main circuit of fluid circulation.
Numerous types of mass flow meters are already known to the professional. They are most often based on a local heating of the passing fluid in the capillary tube, and a measure of the variation in the resistance of the resistive components as a function of the temperature, the aforementioned measurement being representative of the flow of the gas in the tube, and thus of the flow rate. The measurement resistances are most often simply wound around the insulated capillary tube (U.S. Pat. No. 3,938,384).
In the patent entitled DIRECTION SENSITIVE FLOW-RATE INDICATOR (EP 0313 120), Bronkhorst presents a flow indicator device sensitive to the flow of gas in two perpendicular directions by the use of two thermocouples placed in orthogonal directions on a substrate.
In another application, MASS FLOW METERS WITH TEMPERATURE SENSORS (EP 0395 126 B I), Bronkhorst proposes a geometry of the tube having a very elongated U, and equipped with a series of thermocouples placed symmetrically and a central heating resistance in two parts, possibly with Peltier cooling components, for the stage of problems of errors in measurement associated with a circulation of air to the outside of the sensor or internal convection to the capillary tube.
There are many other documents, patents or articles in scientific journals involving thermocouples designed to be integrated into mass flow meters.
The invention presented here proposes a new process for constructing the thermocouple.
According to a second goal of the invention, the process for constructing the device makes it possible to manufacture thermocouples having known characteristics in a precise and reproducible manner.
The device which is the object of the invention is thus a process for constructing the micro-thermocouple sensor designed to be incorporated into a mass flow meter for the circulation of gaseous fluids, comprising the following steps:
deposit of an insulating layer on the sensor tube,
then the deposit of the components of the thermocouple,
then annealing of the sensor,
then the mounting of a heating element on the tube treated in this manner.
Preferentially:
the deposit of the insulating layer on the sensor tube is done to several microns by electron gun,
the deposit of the thermocouple components is also carried out by electron gun, obliquely through the nickel masks, at a residual pressure lower than 10xe2x80x2 Torr, at a thickness of several thousand Angstroms; and
annealing of the capillary tube is done for one hour.
These steps make it possible to construct a thermocouple set upon a capillary tube, with a high degree of manufacturing precision and the final characteristics of the thermocouple, and an excellent manufacturing reproducibility.
According to one particular embodiment, the heating component for the thermocouple for the mass flow meter takes the form of a winding of a filament in an alloy of nickel (75%) and chromium (25%), having a diameter of several tens of microns.
According to another embodiment, the heating element takes the form of a c sleeve on which the CMS resistors are mounted.
According to yet another embodiment, the heating element is deposited on the tube by electron gun, the material of this element is a nickel chromium alloy, and the resistor, mounted in the central region of the thermocouple, includes a central region and two contacts at its ends, and two successive stages of deposition are carried out, with the nickel masks adapted to the different geometries of the resistor and the contacts.
According to yet another embodiment, the heating element includes one thermocouple materials and the heating effect is obtained by the Joule effect in an alternative scheme.
These devices correspond to the embodiment variations and make it possible to obtain either devices which are more economical to manufacture, or devices which are very precise, according to the requirements.
The description which follows, made with regard to the attached drawings in the goal of explaining and in no way limiting, makes it possible to understand the advantages, goals, and characteristics of the invention.